Source multiplexing in lithography

ABSTRACT

An illumination system for an extreme ultraviolet (EUV) lithography system may include multiple sources of EUV light. The system may combine the light from the multiple sources when illuminating a mask.

BACKGROUND

[0001] The progressive reduction in feature size in integrated circuits(ICs) is driven in part by advances in lithography. ICs may be createdby alternately etching material away from a chip and depositing materialon the chip. Each layer of materials etched from the chip may be definedby a lithographic process in which light shines through or reflectedfrom a mask, exposing a photosensitive material, e.g., a photoresistafter imaging through projection optics.

[0002] The ability to focus the light used in lithography, and hence toproduce increasingly smaller line widths in ICs, is a function of thewavelength of the light used. Current techniques may use light having awavelength of about 193 nm. The use of “soft” x-rays (wavelength rangeof λ≈10 nm to 20 nm) in lithography is being explored to achieve smallerdesired feature sizes. Soft x-ray radiation may also be referred to asextreme ultraviolet (EUV) radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003]FIG. 1 is a perspective view of an illumination system for anExtreme Ultraviolet (EUV) lithography system.

[0004]FIG. 2 is a plan view of an array of hexagonal mirrors in amulti-element pupil.

[0005]FIG. 3 is a flowchart describing a method for imaging a maskpattern on a wafer using multiple sources of illumination.

[0006]FIG. 4 is a light combining section of an illumination system.

[0007]FIG. 5 is a flowchart describing an alternative method for imaginga mask pattern on a wafer using multiple sources of illumination.

[0008]FIG. 6 is a perspective view of a scanning reticle receiving lightbeams from multiple sources of illumination. The quality of the diagramshas dropped and needs to be fixed.

[0009]FIG. 7 is a light combining section of an alternative illuminationsystem.

[0010]FIG. 8 is a flowchart describing a method for multiplexing lightfrom multiple sources.

[0011]FIG. 9 is a perspective view of a light combining section of anillumination system utilizing rotating mirrors.

DETAILED DESCRIPTION

[0012]FIG. 1 illustrates an illumination system 100 for a lithographysystem. In an embodiment, the lithography system may be an ExtremeUltraviolet (EUV) lithography system. EUV lithography is a projectionlithography technique which may use a reduction optical system andillumination in the soft X-ray spectrum (wavelengths in the range ofabout 10 nm to 20 nm).

[0013] The system 100 may include multiple sources of EUV radiation110-112, imaging collectors 115, a multi-element pupil 120, andcondenser optics 125. The optical elements in the system (e.g., theimaging collectors 115, pupil 120, and condenser 125) may be mirrorsmade to be reflective to EUV light of a particular wavelength (typically13.4 nm) by means of multilayer coatings (typically of Mo and Si). SinceEUV is strongly absorbed by materials and gases, the lithography processmay be carried out in a vacuum, and a reflective, rather thantransmissive, reticle mask 130 may be used.

[0014] In an embodiment, the sources 110-112 of soft X-rays may be acompact high-average-power, high-repetition-rate laser which impact atarget material to produce broad band radiation with significant EUVemission. The target material may be, for example, a noble gas, such asXenon (Xe), condensed into liquid or solid form. The target material mayconvert a portion of the laser energy into a continuum of radiationpeaked in the EUV. Other approaches may also be taken to produce the EUVplasma, such as driving an electrical discharge through the noble gas.

[0015] The system 100 may combine the illumination from the multiplesources 110-112 such that the light from the sources overlap at the sameimage plane, e.g., the mask plane 130. This may increase the availablepower of the system above that available with a single source. Forexample, the sources in a multi-source EUV lithography system maygenerate about 35 watts individually, but may provide a power output of70 watts or more when combined.

[0016] The multi-element pupil 120 may include an array of hexagonalmirrors. FIG. 2 shows a coordinate system for the hexagonal mirrors inan array 200. Elliptical mirror sections may be used as imagingcollectors 115. Each source may have six associated elliptical mirrorsections. One foci of each elliptical mirror section may be at one ofthe sources 110-112, and the second foci of each elliptical mirrorsection may be at the center of one of the hexagonal mirrors in thepupil array 120.

[0017] The designation “Source: x, y” in FIG. 2 identifies the source(x) the hexagonal mirror is imaging and the number of the ellipticalmirror section (y) associated with source (x) that the hexagonal mirroris imaging light from. For example, the hexagonal mirror 205 with thedesignation “Source: 2, 3” images light from elliptical mirror sectionnumber 3 focusing light from source 2. The central hexagonal mirror 210may receive no light. The distance “r” on the axes refers to thedistance from the center of hexagonal mirror 210 to the position 215 atthe center between the vertices of three adjoining hexagonal mirrors.The center to vertices distance for a hexagonal mirror may be about 0.9r.

[0018]FIG. 3 is a flowchart describing a method 300 for imaging a maskimage onto a wafer using multiple sources of radiation. The ellipticalmirror sections may create eighteen source images, e.g., six images ofeach of the three sources 110-112 (block 305). Each of the eighteensource images may be reflected onto one of the hexagonal mirrors in thearray 200, providing eighteen source images at the pupil 120 (block310). The position and tilt of the hexagonal mirrors in array 200 may beselected such that the central rays of the source images hitting thehexagonal mirrors are reflected parallel to one another (block 315).

[0019] The condenser optics 125 may produce a transformation of theimages at the pupil at the mask plane (block 320). The effect of thetransformation may be that light from all positions on the hexagonalmirror array 200 with the same angle arrive at the same position at themask plane but at interleaved angles. In addition, light leaving thearray 200 from different angles may arrive at the mask plane 130 atdifferent positions. In this manner, the central rays of the sourceimages leaving in parallel from the array 200 may focus to a point atthe center of the mask plane at interleaved angles. The images mayoverlap and the illumination from the multiple sources 110-112 maycombine at the mask plane (block 325).

[0020] The radiation from the condenser 125 may be directed onto themask 130. The mask may include reflecting and absorbing regions. Thereflected EUV radiation from the mask 130 may carry an IC pattern on themask to a photoresist layer on a wafer. The entire reticle may beexposed onto the wafer by synchronously scanning the mask and the wafer,e.g., by a step-and-scan exposure operation. Light from the mask isimaged on to the wafer using projection optics.

[0021] The arrangement of the hexagonal mirrors in the array shown inFIG. 2 may cause the reflected source images to interleave in angle in away that prevents variations in the power or intensity from any onesource from substantially changing the net weighted position of theillumination at the pupil.

[0022] A consideration in designing optical systems is etendue. Etendueis a conserved, invariant quantity in an optical system that may beexpressed as

NA ² ×A=constant

[0023] where NA is the numerical aperture of the radiation incident at asurface of area A. Etendue may represent a measure of the maximum beamsize and solid angle that can be accepted by an optical system.

[0024] The system may be designed such that the combined etendue of thesources 110-112 may be less than or equal to the etendue accepted by theproduction optics. If the etendue is consumed by one of the sources,another source image may not be able to be interleaved at the imageplane.

[0025] In an alternative illumination system 400, a reflective mask 405,or reticle, may be illuminated by light from multiple sources 410-411 ofEUV radiation, as shown in FIG. 4. The surface of the reticle 405 maycontain the pattern to be imaged on the wafer. In an embodiment, anilluminator 415 may use an optical element, such as a corner mirror 420,to combine the light from the EUV sources 410-411.

[0026] The lithography system in which the illumination system 400 isutilized may be a scanning system. In a scanning system, the reticle andthe wafer may be scanned simultaneously under the illumination. Thereticle and the wafer may be mounted on sliding assemblies. The reticlemay be illuminated with a rectangular beam of light which scans acrossthe patterned area as the reticle is moved in a scanning direction. Inan embodiment, a reduction ratio demagnification in the scanning systemmay be 4×. In such a system, the reticle may travel at a speed fourtimes faster than that of the wafer in order to have the image overlapproperly.

[0027]FIG. 5 is a flowchart describing a method 500 for illuminating ascanning reticle using multiple sources of radiation. As shown in FIG.6, light beams 610 and 611 from the sources 410 and 411, respectively,may be directed onto the reticle 405 substantially adjacent to oneanother in the scanning direction 620 (block 505). The reticle 405 maybe scanned under the illumination (block 510) so that each part of thepattern receives the same amount of integrated energy from the twobeams. The illumination may be begun before the beginning of the patternand stopped after the end of the pattern. The light beams may bereflected from the reticle 405 onto the image plane such that thepattern image is scanned on the wafer as the reticle is scanned (block515). A photoresist layer on the wafer may integrate the energy fromboth sources (block 520).

[0028] The total etendue of the system may set the limit on the numberof sources which may be employed in the system.

[0029] As described above, EUV light may be strongly absorbed by manymaterials, including optical elements in the system. In an embodiment,the amount of light reflected from reflective surfaces in an EUVlithography system may be about 67%. The inclusion of the corner mirror420 in the system may increase losses in EUV energy in the optical pathdue to absorption by the added mirror 420.

[0030] In an alternative embodiment, the use of an additional opticalelement, e.g., the corner mirror 420, in the optical path may beavoided. Light beams 701-702 from multiple sources 0.705-706,respectively, may be directed to a pupil 710 at different angles so thatthey overlap at a position 720 on the transform plane at the pupil, asshown in FIG. 7. As described above, a position at the pupil 710 maycorrespond to an angle at the image plane at the mask and an angle atthe pupil may be transformed to a position at the image plane 715. Theangles may be selected such that the light beams arrive at the imageplane in positions 725 and 730, which are parallel and adjacent to eachother.

[0031] In another embodiment, light from multiple sources may bemultiplexed in time. FIG. 8 is a flowchart describing a method 800 formultiplexing light from multiple sources. As shown in FIG. 9, two ormore EUV light sources 900-904 may be focused at the same focal point905, but at different angles (block 805). The light from the multiplesources may be directed to the focal point 910 sequentially at arelatively high repetition rate, e.g., several kilohertz (block 810). Aset of mirrors 910 on a rotating base 915 may be positioned under thepoint of focus 905 synchronously with the repetition rate of the sourcesto align all of the reflections to the same optical path 925 (block815). The mirrors 910 may be angled to direct the light from differentsources arriving at different angles along the optical path 920. Anumber of different sets of mirrors may be rotated on the base to reducethe rate at which the base must rotate. For example, in the system shownin FIG. 9, five separate sets of five mirrors are rotated under the fivesources 900-904. Alternatively, a single moving mirror may be used, butmay need to be tilted and tipped at a precise angle and at a precisetime to correctly align the reflections from the different sources.

[0032] A number of embodiments have been described. Nevertheless, itwill be understood that various modifications may be made withoutdeparting from the spirit and scope of the invention. For example,blocks in the flowcharts may be skipped or performed out of order andstill produce desirable results. Also, the illumination system may beused in other lithography systems, e.g., an x-ray lithography system.Accordingly, other embodiments are within the scope of the followingclaims.

1. A method comprising: overlaying a plurality of images from aplurality of sources on a mask during a lithographic imaging operation.2. The method of claim 1, wherein said illuminating the mask with lightcomprises illuminating the mask with Extreme Ultraviolet (EUV) light. 3.The method of claim 1, further comprising: integrating the light fromthe plurality of sources on a layer of photoresist material.
 4. Themethod of claim 1, further comprising: interleaving in angle the raysforming the images overlaid on the mask.
 5. The method of claim 1,further comprising: illuminating a plurality of elements in a pupil withlight from said plurality of sources, one of said plurality of elementsreceiving light from a corresponding one of the plurality of sources 6.The method of claim 5, further comprising: transforming a common angleof incidence of the light reflected onto the mask into a common positionon an image plane.
 7. The method of claim 6, wherein said transformingcomprises combining light from said plurality of elements at the imageplane.
 8. The method of claim 5, further comprising: generating aplurality of images of each of said plurality of sources at a pluralityof condensers; and reflecting light from each of said plurality ofcondensers onto a corresponding on of the elements in said plurality ofelements.
 9. A method comprising: scanning substantially parallel beamsof light from different ones of a plurality of sources across apatterned portion of a mask during a lithographic imaging operation. 10.The method of claim 9, further comprising: forming a first beam of lighton a mask, said first beam of light originating from a first one of aplurality of sources; and forming a second beam of light on the mask,said second beam of light originating from a second one of saidplurality of sources and being positioned substantially parallel to thefirst beam of light, and wherein said scanning comprises moving the maskrelative to said beams of light in a scanning direction so that the maskis illuminated to the second beam of light subsequent to the first beamof light.
 11. The method of claim 10, wherein forming said first beam oflight comprises forming a beam having a length at least as long as alength of a mask pattern on the mask and a width less than a width ofthe mask pattern, and wherein forming said second beam of lightcomprises forming a beam having a length at least as long as the lengthof the mask pattern and a width less than the width of the mask pattern.12. The method of claim 11, wherein the length of the first beam and thelength of the second beam are substantially perpendicular to thescanning direction.
 13. A method comprising: directing light from aplurality of sources to a focal point sequentially at a repetition rate;and reflecting the light from the plurality of sources along an opticalpath including a mask during a lithographic imaging operation.
 14. Themethod of claim 13, wherein said reflecting comprises reflecting lightfrom the plurality of sources from a plurality of mirrors on a rotatingelement, said mirrors being tilted at different angles.
 15. A systemcomprising: a plurality of sources of light; a pupil including aplurality of reflective elements, each element positioned to receivelight from a different one of said plurality of sources and positionedto receive said light in parallel to light received by other reflectiveelements in the pupil; and one or more optical elements positioned totransform an angle of incidence of light at the reflective elements to aposition on an image plane.
 16. The system of claim 15, wherein theplurality of sources comprise a plurality of sources of extremeultraviolet (EUV) light.
 17. The system of claim 15, further comprisinga plurality of condensers associated with a different one of each of theplurality of sources, each of said condensers positioned to reflectlight from the associated source to a corresponding one of thereflective elements.
 18. A system comprising: a mask including apatterned area; a first source of light; a second source of light; abeam shaper operative to shape light from the first and second sourcesinto a first beam and a second beam; an optical element operative toreflect the first beam and the second beam onto the mask at parallelpositions; and a scanning system operative to move at least one of themask and said light beams in a scanning direction.
 19. The system ofclaim 18, wherein the first source and the second source comprisesources of extreme ultraviolet (EUV) light.
 20. The system of claim 18,wherein the optical element is a corner mirror.
 21. The system of claim18, wherein the optical element comprises a pupil operative to receivethe first beam and the second beam at a common position at differentangles of incidence and reflect the light from the first and secondbeams onto the mask at adjacent and parallel positions.
 22. The systemof claim 18, wherein the lengths of the first light beam and the secondlight beam are substantially perpendicular to the scanning direction.23. A system comprising: a plurality of sources of light, each sourceoperative to direct light onto a target position at a repetition rate;and a reflective element operative to direct the light received at thetarget position to an optical path including an image plane at therepetition rate.
 24. The system of claim 23, wherein the plurality ofsources comprise sources of extreme ultraviolet (EUV) light.
 25. Thesystem of claim 23, wherein the plurality of sources are operative toilluminate the target position in sequence.
 26. The system of claim 23,wherein the reflective element comprises a plurality of mirrors on arotating element, said plurality of mirrors including mirrors tilted atdifferent angles.
 27. The system of claim 23, wherein the image planecomprises a mask plane.